During the past decade, plastic bags have replaced paper bags in the United States for the grocery and retail products industries at a rapid pace because of various inherent advantages in plastic bags. For the most part, these plastic bags have been of the T-shirt type which provide laterally spaced handles integrally extending upwardly from opposed sides of an open mouth portion in the top of the bag to provide ease in carrying of the bag by the consumer. Typically, T-shirt bags are used by grocery and retail stores in the form of packs. Each of such packs includes a plurality of bags, typically 50-200. The pack is mounted on a rack for consecutive detachment of the bags from the pack. The rack also holds the bag in an open position for loading by the sales clerk.
A particularly advantageous bag/rack system, the QUIKMATE.RTM. bag/rack system is disclosed in U.S. Pat. No. 4,676,378. This system allows bags to be supported for loading and to be consecutively removed, one at a time, from a bag pack. An improvement of this system is disclosed in U.S. Pat. No. 5,020,750 to Vrooman, et al. which is assigned to the assignee of the present invention. In accordance with the system disclosed therein, each consecutive bag is self-opening. A disengageable adhesive means is provided between consecutive bags so that the rear wall of each bag is connected to the front wall of the bag behind it. As a filled grocery bag is removed from the bag-rag system, the front wall of the next bag is automatically opened. The resistive force provided by the rack arms against the sliding of the bags results in the breaking of the adhesive means connecting consecutive bags so that a filled grocery bag can be removed from the rack without pulling with it, the next consecutive empty bag, thus avoiding a "daisy chain" effect.
Self-opening bag packs which employ a pressure induced adhesive means between consecutive bags have also been commercially used. As commercially marketed by a number of manufacturers, these bags are composed of a low density polyethylene polymer such as low density polyethylene (LDPE) or linear low density polyethylene (LLDPE). A process for manufacturing bags of this type is disclosed in U.S. Pat. No. 5,087,234 to Prader, et al. According to this disclosure, such bags can be made of various polyethylene materials including low, medium, and high density polyethylene, and they are prepared by corona treating a film tube in a layflat condition and thereafter pressure bonding consecutive bags together during the bag mouth cutting process. Specifically, according to the disclosure of the patent, the pressure and cutting action employed to form the bag mouth and handles will cause adjacently facing corona discharge treated cut-edge regions to releasably adhere together.
In general, the phenomenon of corona-induced self-adhesion of polyethylene film is not a new development as far as film processors are concerned. On the contrary, processors continually fought this problem, more commonly known as "blocking" for many years. In fact, most LDPE and LLDPE contain specific amounts of slip and anti-block additives to counteract the "blocking" effect. However, high molecular weight, high density polyethylene (HDPE) which has a substantially greater crystallinity and is a substantially linear polymer does not tend to block, and more often than not does not contain any slip or antiblock additives.
The mechanism of hydrogen bonding in polyethylene film as a result of corona treating is reported by Owens in J. Appl. Polym. Sci. 19, 256-271 (1975). The polyethylene films treated by Owens were LLDPE (the material was reported to have density of 0.926). However, the conditions of heat and pressure which readily caused blocking in corona treated LDPE and LLDPE seem to have little or no effect on HDPE.
Apparently for similar reasons, although the process disclosed in Prader U.S. Pat. No. 5,087,234 can be successfully employed on low density polyethylene materials to form self-opening bag stacks, this process is generally ineffective when used for high molecular weight, high density polyethylene (HDPE) bag stacks. Thus, this process is not successful even when the degree of corona discharge treatment applied to the surfaces of the tubular film is increased in order to induce self-adhesion of the outer surfaces of adjacent bags during the mouth and handle cutting process. Similarly, even when the cutting blade edges are dulled in order to increase the degree of pressure exerted on the bags during the cutting process, self-adhesion of adjacent bags for self opening is not achieved with HDPE.
Accordingly, although easy-open bag stacks of LLDPE and LDPE film bags can be readily provided without the necessity of a separate adhesive layer between the bags, a separate adhesive layer is still required between HDPE bags when these bags are prepared by prior art manufacturing processes. Moreover with low density polyethylene materials, the known processes for forming self-opening bags such as described in U.S. Pat. No. 5,087,234 to Prader do not allow for substantial adjustment of the degree of bonding between adjacent bags or variation of bonding locations.